1. Field of the Invention
The invention provides devices and methods for end and side derivatization carbon containing materials, such as graphite, carbon nanotubes and analogous structures. Also facile methods to attach moieties and nanoparticles on the side walls and both ends of the carbon nanotubes are described. The invention provides hybide materials for materials applications. Materials have improved properties in the areas of tensile strength, Young's modulus, glass transition temperature, chemical resistance, and electrical or thermal conductivity.
2. Prior Art and Overall Description
There is a continuous need for stronger and lighter materials. Also the supply of the materials should be stable for any foreseeable future. Carbon based new materials, such as graphite fiber and carbon nanotubes, offer great promises to fulfill all these goals. Especially carbon nanotubes (CNTs) have highest tensile strength than any other material. Moreover, they are best electrical conductors at ambient temperature. Despite of great promises there are many problems for the utilizations of the CNTs.
Graphite and CNTs have been used as additives in plastics and composites (Chasiotis I, et al., Multiscale Experiments on Graphite Nanoplatelet/Epoxy Composites, SEM X International Congress and Exposition on Experimental and Applied Mechanics, Costa Mesa, Calif. 2004, Odegard G. M., et al., AIAA Journal 43 (2005) 1828). Often improvement in some properties, such as modulus or break stress, is observed (Qian D, et al., Appl. Phys. Lett. 76 (2000) 2868). When the CNTs are chemically coupled with a polymer the improvements can be significant (Blake R, et al., A generic organometallic approach toward ultra-strong carbon nanotube-polymer composites, J. Am. Chem. Soc. 126 (2004) 10226), although modified CNTs were used as an additive (less than 1%) and were not strongly bonded with the bulk. Polymer side chains have been polymerized from carbon nanotube attached catalysts (Dubois P., et al., WO2005012170, Polymer-Based Composites Comprising Carbon Nanotubes as a Filler, Method for Producing Said Composites, and Associated Uses, 10.2.2005)
Graphite is a hexagonal network of carbon atoms, which are covalently bonded. Covalent bonding is a strongest chemical bond, and carbon-carbon bond is very strong, and in addition that bond has double bond character in the graphite. Carbon nanotubes can be imagined to be formed from a long and narrow graphite sheet by rolling that sheet into a tubular form. Thus, the local structure of graphite and carbon nanotubes is very similar, i.e., it consists of hexagonally bonded carbon atoms. Several graphite-like tubes can be concentric forming multi-walled CNTs. The curvature in the CNTs makes them more reactive than the graphite, although the difference is small between very large multi-walled CNTs and graphite. However, many modification methods of this invention are also applicable to graphite. This invention covers all graphite-like or graphite derived materials, although currently CNTs are most preferred starting materials for hybride materials of this invention.
Composites are traditional way to improve properties of an existing material. Composites have relative coarse structure. Also the various components are not generally chemically strongly bonded. When the structural features are in nanoscale, the borderline between a homogeneous material and composite starts to disappear. This is the case especially, if the components are chemically bonded. With nanostructured materials the term “hybrid materials” is preferred. In hybrid materials various types of chemical moieties or particles can be combined. Components include organic, inorganic, polymeric, and biological molecules and particles. Carbon nanotubes or some other graphite like material is one of these components in this invention, while other components are freely chosen from any of the mentioned classes. In the present invention some or all components are covalently attached with the graphitic materials. In that sense the graphitic materials can be considered as a starting material for the hybride materials of this invention (Hybtonites). The end product contains other elements than carbon, and also other structures than tubes. When the CNTs are starting materials, the end products can not be considered to be CNTs any more, but rather hybride tubes, hybride trees, hybride nets, hybride dendrimers, hybride clusters, hybride monolayers, etc. These can be further organized into higher order hierarchial materials, such as fibers, films, and bulk material, collectively Hybtonites. The situation is completely analogous with all chemical processes, in which the starting material and end product are clearly distinct entities. In this regard, the term hybride nanotube will be used to cover all possible hybride materials, in which CNTs have been one starting material. Corresponding acronym is HNT. The name hydride nanotube emphasizes the fact that these materials have significant amounts of other elements than carbon, and their chemical and physical properties have some unique characteristics. More generally hybride nanostructures derived from graphite or other graphite like materials are denoted by an acronym HNG.
In order the CNTs to be made totally only of carbon they should have at both ends half-fullerene caps. In reality the ends are often open either because they were never capped or the CNTs were cut during purification process. When a CNT is cut by sonification or some other method, the carbon atoms at the ends will have dangling bonds, which are extremely reactive. Cutting is typically done in air or water. Accordingly, a lot of oxygen containing small molecules is nearby. Carbon atoms tend to bind to oxygen forming fenolic and carboxylic functional groups. The present invention allows the suppression of oxidation, and performing a myriad of other reactions during cutting of graphitic materials. The formation of oxygen containing functionalities can be virtually prevented at the expense of deliberately chosen reaction. Alternatively, the formation of certain oxygen containing species can be purposefully enhanced by the methods of this invention.
Ultrasonic vibration is commonly used method to accelerate chemical reactions, and especially heterogeneous reactions. An example is the synthesis of oligonucleotides on the surface of micro- and nanosized silica particles so that the diffusion of the reagents is enhanced by ultrasonic vibration. Cutting cellulose under oxidative conditions has been described (Siegel N., et al., U.S. Pat. No. 5,073,216, Method of ultrasonically cutting fibrous materials and devices therefrom, Dec. 17, 1991). Also reactions between functionalized CNTs and appropriate reagents benefit from the ultrasonic vibration. One example is the reaction between epoxy resist SU-8 oxidized CNTs that contains hydroxyl and carboxyl groups (N. Zhang et al., Smart Mater. Struct. 12 (2003) 280). However, the reactions that have been performed in the art have been such that they would happen without ultrasonic vibration. This is a sharp contrast to the present invention, in which the ultrasound actually enables the reaction by activation the graphitic material itself. This is not a minor difference, because thus typically one or more chemical steps will be avoided in the present invention, what is especially important in industrial production.
Ultrasound creates locally very high pressure and temperature points into the reaction mixture. The temperature can be thousand degrees or more in nano- or microscopic volumes. These high temperatures are randomly located in migrating interference points. Ultrasound induced physical modification of the CNTs have been observed (Iijima S., et al., WO03057622, Porous carbon nanostructure and method for preparation thereof, Jun. 17, 2003)
CNTs have highest tensile strength of any material. This statement is true for one CNT. However, there are still problems in producing macroscopic pieces of CNT based materials. CNTs can be used to reinforce existing materials. However, their straight structure and slippery graphite-like surface is not favorable for this purpose, because the material around them can easily slip. The slippage will eliminate most of the reinforcing effect that the CNTs might have. Chemical cross-linking might damage the CNTs, if it is extensive. The present invention provides methods to avoid the slippage without damaging the CNTs significantly.